Method and apparatus for transmitting information

ABSTRACT

An optical communications system comprises a laser ( 10 ) controlled to output a stream of soliton pulses ( 12 ) coupled to a beamsplitter arrangement ( 14, 20, 22 ) which converts the pulse stream ( 12 ) to a stream of pulses ( 24 ) having twice the pulse rate. This stream of pulses ( 24 ) is modulated to form a binary coded signal by modulator ( 24 ) as a series of binary digits each having one of two values in which a series of n optical soliton pulses, where n is an integer&gt;1 is coupled into an optical fibre ( 4 ) at each occurrence of one of the two values. The optical fibre ( 4 ) and repeaters ( 6 ) provide a transmission link to a receiver ( 8 ). The present invention allows one to increase the average power within a bit without having to operate with shorter pulses that would otherwise be necessary with one soliton pulse per bit of prior art systems in order to avoid the limits inherent in single soliton pulse per bit prior art systems. The result is that the ASE noise limit is moved to longer pulses so allowing higher bit rates notwithstanding that there is a greater number of solitons in each bit of data than with single soliton per bit scheme of modulation. The method also reduces GH the jitter and thus opens the window of operation by pushing the G-H limit to longer pulses

This is a continuation of application Ser. No. 08/039,371, filed Apr.20, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of transmitting informationwhere information is represented a series of binary digits (bits) eachhaving one of two values The present invention also relates to atransmitter for generating such information for transmission, atransmission system for conveying such information, and a communicationssystem for communicating such information between parties.

The present invention in its different aspects finds particular, but notexclusive, application to situations in which information is to beconveyed by means of an optical fibre waveguide in which spaced-apart,discrete, optical fibre amplifiers are used to compensate for lossescaused during propagation along the optical fibre.

2. Related Art

For a receiver of a particular bandwidth at the end of such acommunication system to be able to detect transmitted signals within agiven error rate it must receive signals having a signal-to-noise (S/N)greater than some minimum value.

In an optical fibre transmission line with in-line optical fibreamplifiers, noise is generated by amplified spontaneous emission (ASE)in the amplifiers. The total noise generated by the optical transmissionline therefore depends on the number of amplifiers in the line and theASE noise generated by each amplifier.

The ASE noise is a function of the gain of amplifier which is given by

g=0.23L_(S)γ/n dB   (1)

where L₅ is the system length in km;

n is the total number of amplifiers, all assumed the same; and

γ is the system loss in dB/km.

The solution pulses propagating down the optical fibre transmission linewill lose energy and be subject to intermittent amplification. In orderto have propagation in which the distance average power of the pulse isequal to a single soliton power it can be shown that an Nth ordersoliton has to be launched into the optical transmission line where N isgiven by

N²=log(g)/(1−1/g)   (2)

The minimum average power Pmin necessary to achieve an S/N ratiosufficient to give a 10⁻¹⁴ bit error rate is

P_(min)=10⁻⁴Bn[exp(45L₅/n)−1]mw   (3)

where B is the bandwidth in G/bits.

It can be shown that the pulse width required to generated the requiredNth-order soliton pulse of the desired minimum average power P andbandwidth B is given by

t_(ASE)=0.658 N²BD/P   (4)

where D is the dispersion.

Equation (4) puts a constraint on the maximum soliton pulse width, t, toachieve the desired S/N ratio.

Another source of noise which becomes increasingly important at higherbit rates is the Gordon-Haus effect: see J. P. Gordon and H. A. Haus,Random Walk of Coherently Amplified Solitons in Optical FibreTransmission, Optics Lett 11 665-7 (1986).

This effect induces an error due to fluctuations in arrival times,t_(n), which occurs from the combined action of an ASE induced frequencyfluctuation and dispersion. The mean square jitter can be expressed as$\begin{matrix}{{\langle{\gamma \quad t_{a}^{2}}\rangle} = {( \frac{D^{2}L_{s}}{9t^{2}} )( \frac{n_{noise}}{n_{pulse}} )}} & (5)\end{matrix}$

where n's are photon numbers. The error rate due to this effect can becalculated.

The operating nonlinear dynamics imposes a second requirement that thesoliton period be rather longer than the amplifier spacing L_(A), namely

t_(spacing)>(0.3DL₁α)^(½)  (6)

where α is a safety factor of about 10. Once these two conditions ofequations (4) and (7) are satisfied one observes essentiallydistortionless propagation of single pulses over arbitrarily largedistances.

In FIG. 1 is shown a plot of amplifier spacing against pulse durationshowing the three limiting processes (equations 4,5 and 6) for theexample of a 6000 km system length. The G-H effect is the only one whichdepends on bit rate and this is plotted for three bit rates (10, 8 and 5Gbit/s).

To operate a prior art soliton transmission system, t, the soliton pulsewidth, must be less than the G-H limit, less than t_(ASE) and greaterthan t_(spacing).

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amethod of transmitting information as a series of binary digits, eachhaving one of two values, in which a series of n optical soliton pulses,where n is an integer>1, is coupled into an optical fibre at eachoccurrence of one of the two values.

According to a second aspect of the present invention there is providedan optical transmitter for transmitting information as a series ofbinary digits each having one of two values comprising a source ofoptical soliton pulses arranged to provide a series of n optical solitonpulses, where n is an integer>1, at each occurrence of one of the binarydigit values, and optical coupling means for coupling the series ofpulses into an optical fibre.

The present invention allows one to increase the average power within abit without having to operate with shorter pulse that would otherwise benecessary with one soliton pulse per bit of prior art systems in orderto avoid the limits imposed by equations 5 and 6 on single soliton pulseper bit prior art systems. The result is that the ASE noise limit ismoved to longer pulses so allowing higher bit rates notwithstanding thatthere are a greater number of solitons in each bit of data than withsingle soliton per bit schemes of modulation.

The method also reduces G-H jitter and the thus opens the window ofoperation by pushing the G-H limit to longer pulses. The reason for thisis that the pulse will be independently subject to a jitter so that anerror will occur only if both pulses are sufficiently shifted. A simpledelta function model for the pulses given a n^(½) reduction in thejitter.

A potential problem with the multiple pulse per bit modulation scheme ofthe present invention is pulse-pulse (i.e. soliton-soliton) interaction.This is a well known phenomenon which leads to a collapse of the pulsesif they are initially in phase. The total energy per bit is unaltered bythe collapse and so this will not, in principle, affect the systemperformance. It is desirable in such cases that there is no interactionwith the adjacent bit pulses. It is then preferable that each series ofn optical soliton pulses representative of a binary digit is separatedfrom the next series by a time interval greater than the pulses withineach series.

When pulse-pulse interaction is significant it is also desirable thateach pulse in each series of n optical soliton pulses is O or π/2radians out of phase relative to the adjacent pulses in the series.

A convenient way to achieve variations in pulse spacing and phasecontrol for two solitons per bit is a transmitter comprising means forgenerating a stream of optical soliton pulses, a beam splitter locatedto split the stream of optical pulses into two subsidiary streams, apair of reflective means to reflect the two subsidiary streams back tothe beam splitter, the reflective means being positioned such thatpositions of the two subsidiary streams are combined by the beamsplitter to form as an output stream of optical soliton pulses alternateones of the subsidiary streams. Series of pulses greater than two can bereadily produced by multiplexing the appropriate number of suchtransmitters.

The distance of one of the reflective means, for example a mirror, canbe made adjustable to provide equidistantly spaced or paired opticalpulses of the desired phase relationship. Once this phase is fixed thecollapse effect on propagation will not reduce the energy in the bitinterval. The proposed scheme can also operate in the regime of completeoverlap of the pulses in the centre of the bit. This is equivalent tooperating with solitons of the initial form Nsech(t) with N an integerequivalent to the number of pulses per bit. This latter multiple solitonmodulation scheme will have the same benefit as for the increase inaverage power but will not have the benefit of the reduced jitter.

According to a third and a fourth aspect of the present invention anoptical transmission system comprises an optical transmitter opticallycoupled to an optical fibre communications network having two or morespaced apart optical amplifiers and an optical communications systemcomprising the above transmission system and an optical receiver coupledto the optical fibre communications network for receiving thetransmitted optical spliton pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention illustrating its several aspectsand its principle of operation will now be described with reference tothe accompanying drawings of which:

FIG. 1 is a graph of the factors determining the operational limits of aprior art, 6000 km, single soliton pulse per bit, soliton communicationssystem;

FIG. 2 is a graph of the factors determining the operational limit of a6000 km system of FIG. 1 but using a double soliton pulse per bitaccording to the method of the present invention;

FIGS. 3(a), (b) and (c) are diagrammatic representations of a binaryinformation signal transmitted according to the present invention havinguniform and non-uniform pulse spacings respectively, and

FIG. 4 is an embodiment of an optical communications system according tothe present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

FIG. 1 shows the calculated limits of t_(spacing), t_(ASE) and theGordon-Haus limit for 5.8 and 10 Gbits/s data rates for singlesoliton/bit transmission. Transmission is possible for amplifierspacings for which there is a pulse width that is simultaneously lessthan t_(ASE), greater than t_(spacing) and greater than the G-H bit ratelimit. This window of operation is shown shaded in FIG. 1. It can beseen that for large inter-amplifier spacing of say 50 km one is limitedto data rates of up to about 5 Gbits/s.

Referring to FIG. 2, there is shown the calculated limits for the twosoliton per bit transmission of the present invention in which thereduced jitter benefit has not been included as this may not pertainwith soliton-soliton collapse. The effect of increasing the averagepower without having to operate with a shorter soliton pulse can be seento open the operating window to that shown by the shaded area of FIG. 2.Thus an 80 km amplifier spacing can be achieved for data rates under 5Gbits/s or 50 km amplifier spacing at 8 Gbits/s data rate. For closespacing of amplifiers, 10 Gbit/s operation is possible which could notbe achieved previously.

Referring now to FIG. 1, there are shown two modulation schemes of thepresent invention in which an exemplary data stream of binary digits ofFIG. 3(a) have been encoded with two soliton pulses per bit in which thepresence of a pulse represents a value of 1. In FIG. 3(b) the solitonpulses are equidistantly spaced in the bit period. In FIG. 3(c) thesoliton pulse pairs are bunched towards the centre of the bit period sothe time interval between each series of two pulses in a bit is greaterthan the time interval between the pulses within a bit period.

Referring to FIG. 4, an optical communication system comprises anoptical transmitter 2, a 6,000 km optical fibre transmission line withspaced apart erbium fibre amplifiers 6, and an optical receiver 8.

The transmitter 1 comprises a mode-locked, semiconductor laser 10 whichgenerates in a known manner a stream of optical soliton pulses which isdirected at a bulk optic beamsplitter 14. The stream 12 is divided bythe beamsplitter 14 into two subsidiary streams 16 and 18 (with pulseslabelled P1 and P2, respectively) which are reflected back to thebeamsplitter 14 by mirrors 20 and 22. The distance L of the mirror 22from the beamsplitter 14 is adjustable. The distance is set, in FIG. 4,to obtain equidistant interleaving of the subsidiary streams 16 and 18to form an output stream 24 of pulses at twice the rate provided by thelaser 10. The mirror 22 can be adjusted to tune the phases of pulses P1relative to pulses P1.

The output stream 24 is modulated by modulator 16 in response toelectrical signals from a controller 28 modulated with a binaryinformation signal. The data rate is such that two soliton pulses lie ina single bit period.

The modulated soliton pulse stream is then coupled into the opticalfibre transmission line 4, 6 for onward propagation to the receiver 8 bystandard optical means (not explicitly shown).

What is claimed is:
 1. A method of transmitting digital data comprising:providing digital data in the form of bits each having one of twopossible binary digit values; defining a sequence of successive bitperiods for the bits respectively; generating a time series ofsuccessive distinct optical soliton pulses; and encoding each bit havinga predetermined one of said two possible values as a time series of n ofsaid distinct optical soliton pulses during the bit period thereof,where n is an integer>1.
 2. A method as in claim 1 including the step ofgenerating each series of n optical soliton pulses by generating acontinuous stream of optical soliton pulse and modulating the stream as(1/n)th the pulse repetition rate.
 3. A method as in claim 1 in whicheach series of n optical soliton pulses representative of a binary digitis separated from the next series by a time interval greater than thetime interval between consecutive pulses within each series.
 4. A methodas in claim 3 in which each pulse in each series of n optical solitonpulses is out of phase relative to adjacent pulses in the series.
 5. Amethod as in claim 1 in which the optical soliton pulses are generatedby means of a mode-locked semiconductor laster.
 6. An opticaltransmitter for transmitting information as a series of binary digits,each having one of two values, said transmitter comprising: a source ofoptical soliton pulses providing at an output port a series of nseparate optical soliton pulses, where n is an integer>1, at eachoccurrence of one of the binary digit values, and optical coupling meansoptically connected to said output port for coupling or not coupling theseries of n soliton pulses into an optical fibre as a function of saidbinary digit values.
 7. A transistor as in claim 6 wherein said sourceincludes a generating means for generating a continuous stream ofoptical soliton pulses having a predetermined pulse repetition rate andsaid coupling means includes a modulating means to modulate the streamat (1n/th) the pulse repetition rate.
 8. A transmitter as in claim 6 inwhich each series of n optical soliton pulses representative of a binarydigit is separated from the next series by a time interval greater thanthe time interval between pulses within each series.
 9. A transmitter asin claim 8 in which each pulse in each series of n optical solitonpulses is out of phase relative to adjacent pulses in the series.
 10. Anoptical transmitter as in claim 6 in combination with an opticaltransmission system having an optical fiber communications network, saidoptical transmitter being optically coupled to said optical fibrecommunications network which includes two or more spaced apart opticalamplifiers.
 11. A system as in claim 10 in which the optical amplifiersare optical fibre amplifiers.
 12. An optical transmission system as inclaim 10, further including a receiver coupled to the optical fibrecommunications network for receiving the transmitted optical solitonpulses.
 13. A transmitter for transmitting information as a series ofbinary digits, each having one of two values, said transmittercomprising: a source of optical soliton pulses providing at an outputport a series of n optical soliton pulses, where n is an integer>1, ateach occurrence of one of the binary digit values, and optical couplingmeans optically connected to said output port for coupling or notcoupling the series of n soliton pulses into an optical fibre as afunction of said binary digit values, said source means including: asource of optical soliton pulses; a beam splitter located to split thestream of optical pulses into two subsidiary streams; a pair ofreflective means to reflect the two subsidiary streams back to the beamsplitter; the reflective means being positioned such that the twosubsidiary streams are combined by the beam splitter to form an outputstream of optical soliton pulses alternate pulses of which derive fromdifferent ones of the subsidiary streams.
 14. A transmitter as in claim13 in which the distance of at least one of the reflective means fromthe beam splitter is adjustable.
 15. A transmitter as in claim 13 inwhich the reflective means are positioned to provide equidistant spacingof the pulses in the output stream.
 16. A transmitter as in claim 13 inwhich the reflective means are positioned to provide the output streamof optical soliton pulses as pairs of pulses, each pair separated fromthe next pair by a time interval greater than the time interval betweenpulses of each pair.
 17. A method of transmitting digital data in theform of a sequence of bits in successive bit periods, said methodcomprising the steps of: producing a soliton pulse series; deriving fromthe soliton pulse series at least first and second subsidiary streams ofsoliton pulses, each of said subsidiary streams corresponding to thesoliton pulse series; combining the subsidiary streams to produce acarrier pulse stream wherein each of the bit periods includes solitonpulses from each of the subsidiary streams; and modulating the carrierstream according to the digital data, whereby successive bits of thedigital data are represented by the presence or absence of a pluralityof soliton pulses during each successive bit period.
 18. Apparatus fortransmitting digital data in the form of a sequence of bits insuccessive bit periods, said apparatus comprising: a digital datasource; an optical source producing an optical soliton pulse series;means for deriving from the soliton pulse series at least first andsecond subsidiary streams of soliton pulses, each of said subsidiarystreams corresponding to the soliton pulse series; combining means forcombining the subsidiary streams to produce a carrier pulse streamwherein each of the bit periods includes soliton pulses from each of thesubsidiary streams; and modulator means for modulating the carrierstream according to the data from the digital data source, wherebysuccessive data bits of the digital data are represented by the presenceor absence of a plurality of soliton pulses during each successive bitperiod.